1. Field of the Invention
The present invention relates to a method for producing an organic thin film transistor and an organic thin film transistor produced by the method.
2. Description of Related Art
Recently, as switching elements for driving pixels of a flat panel display device and an electronic paper, which are next-generation high-quality low-price devices, organic thin film transistors (OTFTs) draw attention.
An organic thin film transistor is basically of the same structure as that of a silicon thin film transistor. However, while the silicon thin film transistor has a semiconductor active layer made of silicon, the organic thin film transistor has a semiconductor active layer made of an organic material. Such organic thin film transistors can be produced by an ink jet process, a printing process or the like without using a vacuum device, and accordingly can be produced easily and at low cost, compared with silicon thin film transistors. Also, the organic thin film transistors have advantages that they are not broken easily by an impact and that they bend and fold easily, and the organic thin film transistors are suited to be used for an electronic circuit board. Also, when elements must be arranged in a large area and when a process is performed under low temperature, using the organic thin film transistors is effective. Therefore, the organic thin film transistors are expected to be used as matrix driving elements of a large display, driving elements of an organic EL and driving elements of an electric paper, and many makers have been developing organic thin film transistors.
Conventionally, a film pattern of a semiconductor element is formed by photolithography and etching in the following way. A resist is coated entirely on a substrate, and the substrate is pre-baked. Thereafter, the substrate is irradiated with ultraviolet rays via a photo-mask, and a resist pattern is formed by development. Then, etching is performed by using the resist pattern as a mask so that a film (made of a semiconductor material, an insulating material or a conductor material) in an unnecessary part can be removed. Thus, a film pattern is formed.
In a production process of an organic thin film transistor with a patterned organic semiconductor layer, patterning of the organic semiconductor layer is possible by the above-described conventional photolithography and etching, but as mentioned, it is easier to perform patterning by a printing process or an ink jet process. In these processes, the ink jet process is a process wherein an arbitrary pattern can be formed without a mask and an organic semiconductor material needs to be coated only on necessary parts, that is, an organic semiconductor material can be used efficiently (refer to, for example, Japanese Patent Laid-Open Publication No. 2004-221562 (Reference 1)).
The reference 1 says that the pattern to be formed by the ink jet process may be of any shape and describes a dot pattern as an example. The reference 1 also says that masking may be performed. However, the reference 1 is silent about the specific shape of an electrode on which a semiconductor material is to be dropped and the details of dropping.
When an organic semiconductor layer is formed by dropping, the organic semiconductor layer must be formed into a shape matching with the shape of an electrode on which the semiconductor layer is to be formed. In order to form an organic semiconductor layer in the desired shape more exactly, it is necessary to make more drops by use of a dropping device with smaller nozzles. As the number of drops is increasing, the time for dropping becomes longer, which causes a problem that the productivity becomes lower. Also, a semiconductor layer formed of a large number of drops has unevenness in the layer thickness due to overlap of drops. This may change the characteristics, which may cause trouble.